1. Field of the Invention
The present invention relates to modulators. More specifically, the invention relates to balanced modulators (those having no carrier component in the output waveform), especially balanced quadrature amplitude modulation (QAM) modulators.
2. Related Art
In U.S. Pat. No. 4,804,931 (which is incorporated herein by reference in its entirety), a digital amplitude modulator-transmitter is described. An amplitude modulator-transmitter embodying the teachings of the '931 patent is illustrated in FIG. 1.
The amplitude modulator 30 includes a predetermined number of quadrature hybrid power devices 32.sub.1 -32.sub.n. The quadrature hybrid power devices 32.sub.1 -32.sub.n are configured as combiners, and are arranged in cascade so that the output of one combiner is the first of two inputs of a successive combiner. In the exemplary illustration of FIG. 1, only four combiners are shown for ease of understanding.
The construction and operation of a quadrature hybrid device is described in detail in the '931 patent, and need not be described here. However, for purposes of the present discussion, a quadrature hybrid combiner may be implemented as a four-port device having first and second inputs, one output, and one isolated port. The first and second inputs receive two signals of a given amplitude that are 90.degree. apart in phase. The input signals are combined to provide an output signal having an amplitude that is the sum of the amplitudes of the input signals. As shown in FIG. 1, ports 1 and 4 represent the first and second input ports. Port 3 represents an isolation port to which is attached a resistor representing a dummy load 34. Finally, port 2 defines the output port of each of the combiners.
An analog signal source 36, such as a microphone or a video generator, produces an analog signal that passes through an analog-to-digital converter (A/D converter, or ADC) 38. The digital output of the A/D converter appears on data lines 40.
As an example, there are four data lines, so that data consists of four-bit words. However, it is contemplated that 12- or 16-bit words may be employed to improve the quality of the signals being transmitted. Further, it is contemplated that a digital input signal may be fed directly to gates 42 without the need for A/D converter 38, should the modulating signal already exist in binary form.
Each of the bits, from the least significant bit (LSB) to the most significant bit (MSB), controls a respective gate 42.sub.1 -42.sub.n. Each of the gates is connected in series with a corresponding amplifier 44.sub.1 -44.sub.n. The series combinations receive an RF signal from an RF signal generator 46, and provide respective second inputs to the combiners.
The output port of the LSB combiner 32.sub.1 represents the least significant bit of the digital word describing the instantaneous value of the signal to be modulated. The second input of the combiner 32.sub.2, adjacent the LSB combiner 32.sub.1, represents a value that is twice as significant as that of the second input of the LSB combiner 32.sub.1. Similarly, the second input of each successive combiner represents a value twice as significant as that input to the immediately preceding combiner.
Thus, the output of each combiner represents a binary weighted power signal. In particular, the signal appearing at the output of the MSB combiner 42.sub.n represents the sum of the power signals fed into the various combiners. This output signal is fed for transmission to a load, typically an antenna with conventional filtering.
The modulator-transmitter in the '931 patent has many advantages, such as its ability to provide pseudo-continuous amplitude modulation at any modulation index between 0 and 1, at any carrier frequency, using any one of the wide variety of classes of amplifiers. However, it is not a balanced modulator, as a carrier component is present in the output waveform, Thus (assuming a sinusoidal modulating signal of angular frequency .omega..sub.m), corn), the digital amplitude-modulator described in the '931 patent provides an output signal which may be represented as: EQU S.sub.AM (t)=A(1=cos .omega..sub.m t)(cos .omega..sub.c t) (1)
where:
S.sub.AM (t) is the output of the amplitude modulator;
A is a constant;
.omega..sub.m is the modulating signal angular frequency;
.omega..sub.c is the carrier signal angular frequency; and
t is the variable for time.
As will readily be appreciated by those skilled in the art in light of the '931 disclosure, a carrier frequency component, and symmetrical upper and lower sidebands, are present in the output signal,
For various reasons appreciated by those skilled in the art, suppression of the carrier is often desirable. In particular, in formats which may be used for high definition television (HDTV) and digital cellular telephone systems, suppression of the carrier is required.
Generally, a balanced modulator provides an output having no carrier, and may be expressed by the formula: EQU S.sub.BAL (t)=A(cos .omega..sub.m t)(cos .omega..sub.c t) (2)
This formula assumes that the modulating waveform is sinusoidal in form with an angular frequency of .omega..sub.m. More generally, a balanced modulator receiving an arbitrary modulating waveform a(t) having no DC component provides an output that can be described by the formula: EQU S.sub.BAL (t)=A(a(t)) cos .omega..sub.c t (3)
However, the '931 patent does not describe a manner in which a balanced modulator may be implemented. It is to meet this need that the present invention is directed.
Various patents are directed to modulators. For example, U.S. Pat. No. 5,153,536 (Muller) discloses a QAM modulator having an arrangement of double modulators and partial modulators. U.S. Pat. No. 3,757,222 (Oberbury) discloses a single sideband generator involving selective switching of transmission line links. U.S. Pat. No. 4,635,004 (Ishigaki) discloses a single sideband generator in which quarterwave-shifted signals are alternately input to a balanced modulator. U.S. Pat. No. 4,717,894 (Edwards et al. ) discloses a method of calibrating a vector modulator involving in-phase and quadrature components. Finally, U.S. Pat. No. 4,068,100 (Thompson) discloses an AM stereo system in which opposite-polarity half-cycles of a carrier are separately modulated with a different channel. However, none of the existing art is believed to provide all the advantages of the present invention.